1. Field of the Invention
The present invention relates in general to filling or plugging of drill holes. In particular, the present invention relates to an improved method for plugging an abandoned drill hole within the earth and a capsule for use therewith.
2. Description of the Related Art
It has been well known to provide shallow (on the order of 30 m or 100') drill holes within the earth for a variety of purposes. Such holes are typically formed with a standard seismic drill. In forming such holes, it is common for the drill to pass through several layers having different compositions. For example, the drill may pass through hard compacted soil, clay, loose sand, and other typical geologic material. Additionally, it is also fairly common for the drill to pass through one or more water bearing layers during formation of the hole. Such water bearing layers may be a saline water source or a fresh water aquifer. Unfortunately, the passage of the drill hole through such aquifers may cause damage to the aquifer.
In particular, a fresh water aquifer may "leak" downward through the hole into a fracture or uncharged zone, causing loss of water from the aquifer. A drill hole extending between a saline water source and a fresh water aquifer may allow commingling of these water supplies, damaging both. Additionally, contamination from the surface, such as surface rain water passing downward through the hole into a fresh water aquifer, may cause damage.
To overcome this problem it has been known to plug the drill holes, at least to a level above the water sources, with high quality coarse ground chemically unaltered sodium bentonite (hereafter bentonite). The bentonite will swell greatly upon hydration creating a high quality and reliable plug. While properly placed bentonite plugs are quit reliable, it has been a problem to place the bentonite in the proper position within the drill hole.
A first and simple method for placing the bentonite is to simply pour a small granular form of dry bentonite into the drill hole from the surface. The bentonite will then fall downward through the drill hole, filling the hole from the bottom upward. However, where the drill hole passes through unconsolidated material, such material may form a cave in at the sides of the drill hole, forming a plug at a position spaced above the bottom of the hole. In such cases the small granular bentonite will simply fill the hole from the plug upward and not pass downward to the bottom of the hole to fully seal the water source. Additionally, this pour filling method is not possible in drill holes passing through high volume artisan flows, or in drill holes using a dug pit (i.e. where a bentonite slurry has been employed to maintain wall integrity in the hole).
The conventional form of bentonite poured into the hole is formed of small granular particles having a diameter of no greater than 3/8 inches. Such small material has proven ineffective when poured into holes having high fluid flow rates therethrough and when poured into holes retaining a high liquid level (i.e., a long distance between the hole bottom and liquid level). As the small granular material passes through the liquid, it begins to hydrate and swell. Granular bentonite having a diameter of no greater than 3/8 inches swells quickly and plugs the hole prior to reaching the bottom. Hence, conventional small granular material is ineffective for filling deep holes. Additionally, the conventionally sized granular bentonite falls through the liquid in the hole in an unconcentrated state. Each granular particle is effectively afforded the entire cross-sectional area of the hole within which to expand. Sodium bentonite will continuously expand until it is restrained by its surroundings or starved for water. Once the bentonite expands to a size several times its dehydrated size, the conventionally sized bentonite granule loses its solid structure and turns to a slurry liquid state. Once sodium bentonite hydrates to the point that it turns to a slurry liquid, the granule becomes ineffective at plugging holes.
Past systems that use the conventional sized bentonite particles have prevented degradation to this slurry state by filling the hole with dehydrated granular particles before each individual particle is allowed to expand substantially. To do so, the granules are poured into shallow holes or holes having very little liquid standing therein. In shallow holes, conventionally sized particles collect in the bottom of the hole before expanding substantially. However, when conventionally sized granular bentonite is poured into deep holes and through deep liquid levels, each individual particle turns to a slurry state before reaching the bottom of the hole and collecting with the other falling particles.
A second and more reliable method is to insert a conduit into the drill hole and pass a slurry of bentonite through the conduit while slowly withdrawing the conduit. For example, Kitanaka U.S. Pat. No. 5,013,191 discloses a special auger which is rotated in the normal manner to drill the hole, and then is fixed against rotation while the bentonite slurry is passed through a central hole in the auger and the auger is withdrawn. While this method is effective, it requires the use of a special and expensive auger.
An alternative slurry/conduit method consists of simply inserting a standard 11/2 inch PVC pipe into the drill hole and passing the slurry through this pipe. While this method does not require the use of a special auger, if the hole has been plugged as noted above, the method requires an initial step of drilling with an auger to clear the plug prior to inserting the pipe.
Moreover, problems have been encountered with the above systems when using a mixture of heavy bentonite gel water slurry. The slurry mixture is used while drilling the holes to keep the walls of the drill holes from sluffing inward, thereby avoiding the need to reconstruct sluffed areas within the hole. After abandonment, the slurry stands within the hole. The density of the slurry is sufficiently close to the density of conventional granular bentonite which is poured directly into the hole, that the slurry holds the granular bentonite in suspension proximate the top section of the hole. Thus, when the granular bentonite is poured into the hole, it does not sink to the bottom, and thus does not plug the hole from the bottom up.
Moreover, the foregoing systems are ineffective when used with wet auger drilled holes which utilize water injected from the surface downward into the hole. While drilling the hole, the agitation of the auger stem, when combined with the injected water, creates a heavy native mud material that remains within the hole after drilling is completed. The density of this mud is relatively high, with respect to that of bentonite granular material, and thus holds the bentonite granular material in suspension at the top of the hole.
A need remains within the industry for an improved method and apparatus for plugging abandoned drilled holes. It is an object of the present invention to meet this need.